Field emission of electrons into vacuum from suitable cathode materials is currently the most promising source of electrons in vacuum devices. These devices include flat panel displays, klystrons and traveling wave tubes used in microwave power amplifiers, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. The most promising application is the use of field emitters in thin matrix-addressed flat panel displays. See, for example, J. A. Costellano, Handbook of Display Technology Academic Press, New York, pp. 254 (1992), which is incorporated herein by reference. Diamond is a desirable material for field emitters because of its low voltage emission characteristics and robust mechanical and chemical properties. Field emission devices employing diamond field emitters are disclosed, for example, U.S. patent application Ser. No. 08/361616 filed by Jin et al. Dec. 22, 1994. This application is incorporated herein by reference.
A typical field emission device comprises a cathode including a plurality of field emitter tips and an anode spaced from the cathode. A voltage applied between the anode and cathode induces the emission of electrons towards the anode.
A conventional electron field emission flat panel display comprises a flat vacuum cell having a matrix array of microscopic field emitters formed on a cathode of the cell (the back plate) and a phosphor coated anode on a transparent front plate. Between cathode and anode is a conductive element called a grid or gate. The cathodes and gates are typically skewed strips (usually perpendicular) whose intersections define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (400-3,000 eV) to the emitted electrons.
The anode layer is mechanically supported and electrically separated from the cathode by pillars placed sparsely so as not to drastically reduce the field emission areas of the display. In order to withstand the high voltage applied to the anode for phosphor excitation, the pillar material should be dielectric and should have high breakdown voltage.
One of the limiting factors in the display performance in the fiat panel, field emission display (FED) is the allowable maximum operating voltage between the cathode emitter and the anode. The measured efficiency for typical ZnS-based phosphor, (e.g. the P22 red, green, and blue, as commercially available from GTE) increases approximately as the square-root of the voltage over a wide voltage range, so a field emission display should be operated at as high a voltage as possible to get maximum efficiency. This is especially important for portable, battery-operated devices in which low power consumption is desirable. The applicants have also found that the electron dose that phosphors can survive without substantial degradation of their luminous output similarly increases with operating voltage. It is not generally recognized that the combination of these two effects makes it especially advantageous to operate at high voltage. The display needs to produce the same light output, irrespective of its operating voltage. Since the efficiency improves at high voltage, less total power must be deposited on the anode. Further, since the power is the anode voltage times the current, the current required to maintain a constant light output decreases even faster than the power. When this is combined with the above-mentioned increase in dose required to damage the phosphor, the lifetime is found to be a strongly increasing function of the voltage. For a typical phosphor, we anticipate that changing the operating voltage from 500 V to 5000 V would increase the device's operating lifetime by a factor of 100.
Most practical field emission displays require integrated dielectric pillars to keep the substrate and screen separated. Without these pillars, the pressure difference between a normal atmosphere outside and vacuum inside will flex the anode and the cathode surfaces together. Because of the insulator breakdown in high electrical fields, these pillars put limitations on the voltage that can be applied to the display, and consequently limit the phosphor efficiency and thus the power consumption. The voltage limitation arises because it is necessary to avoid electric discharges along the surface of the pillars.
There is a substantial amount of knowledge on surface breakdown on insulators in vacuum, see a review paper by R. Hawley, Vacuum, vol. 18, p. 383 (1968). For insulator surfaces oriented parallel to the electric field, typical electric fields at which breakdown occurs seem to be no better than 10.sup.4 V/cm (e.g., 5000 V across a 5 mm length). This is dramatically lower than the 1-10.times.10.sup.6 V/cm that most solid insulators will support through the bulk. Smaller dielectric objects will support larger electric fields, for example, 200 .mu.m high pillars will typically support about 2-5.times.10.sup.4 V/cm, but the overall voltage (which is field times height) is still a monotonic function of height.
Since field emission displays with ZnS-based phosphors are desirably operated at 2000 V or more (even more desirably at 4000 V or more), a straight-walled pillar would have to be 0.5 mm-1 mm tall (allowing for a safety factor of 1.5). Such tall pillars lead to difficulties in keeping the electrons focussed as they travel between emitter and the phosphor screen.
The applicants are not aware of any literature that discusses the effects of electron bombardment on dielectric breakdown, but it seems likely that it will decrease the breakdown voltages further, and thus require yet taller pillars.
If we consider an insulating surface in a vacuum containing a few electrons, the insulator surface will generally become charged. The sign of the charge is not necessarily negative. Incoming electrons can knock electrons off the insulator, a process known as secondary emission. If, on average, there is more than one outgoing electron per incoming electron, the insulator will actually charge positively. The positive charge can then attract more electrons. This process doesn't run away on an isolated block of insulator, because the positive charge eventually prevents the secondary electrons from leaving, and the system reaches equilibrium.
However, if we put the insulator between two electrodes and establish a continuous voltage gradient across the insulator, the secondary electrons can always hop toward the more positive electrode. One can get a runaway process where most of the insulator becomes positively charged (to a potential near that of the most positive electrode) so that the voltage gradients near the negative electrode becomes very strong. These stronger gradients can lead to field emission from the negative electrode, and another cycle of charging and emission. This process can lead to the formation of an arc across the surface long before the insulator would break down through the bulk. Accordingly there is a need for novel and convenient methods for producing and assembling a pillar structure with desirable geometrical configurations and dielectric properties.